Description of Invention
The present invention relates to lubricant additives. The present invention also relates to methods of forming the lubricant additives. Furthermore, the present invention relates to the use of the lubricant additives.
Background
There is a continuous need for improved engine performance and reduced emissions.
Lubricating oils and greases are engineered to function over a broad range of temperatures and loading conditions. Modern engines operate at higher temperatures, speeds and pressures than previous engines, and therefore require lubricants capable of handling these harsher conditions. Reliable performance in extreme conditions is also necessary in emergency and combat situations.
In automotive engines, the temperature at the surfaces of critical tribological components can easily reach 200° C., while asperity contacts can generate ‘flash temperatures’ up to 1000° C. of microsecond durations. The extreme pressure and temperature in the contact zones can lead to plastic deformation, wear away mating surfaces, and catalyze undesirable chemical reactions which damage the surfaces and lubricants.
Conventional lubricants and oils undergo degradation via three main pathways: scission, thermolysis, and oxidation. The high temperatures and pressures of typical engines create an environment that is hostile to the long molecular hydrocarbon chains found in lubricants. These degradation pathways lead to irreversible reductions in viscosity and the generation of oil-insoluble acids and salts that corrode surfaces and form performance-damaging sludges. The additive packages used in modern lubricants contain compounds designed to preserve the longevity of the lubricant. These include friction modifiers, viscosity modifiers, dispersants, corrosion inhibitors, and anti-oxidants.
Solid lubricants, applied either as a surface coating or as a lubricant additive, are well-suited for high-temperature operation. Most solid lubricants, such as graphite and molybdenum disulfide, have a strong 2D lamellar structure and weak intracrystalline interactions, enabling low-friction sliding of basal planes under shearing forces.
The ductility of soft metals can also be utilized in lubrication. The low shear-strength of metallic films can form a smooth “glaze layer” on tribosurfaces that lubricates sliding contact, and the low reactivity of noble metals enables this mechanism to function at extreme temperatures.
Silver coatings in contact surfaces have demonstrated friction and wear improvement in temperatures ranging from 25-750° C. Silver nanoparticles have also been shown to greatly increase surface fatigue life, decrease friction and wear, and work synergistically with other lubricant additives. However, silver nanoparticles are costly to produce, difficult to suspend in oil, and often require a surfactant to prevent the particles from agglomerating.
An alternative method for the delivery of lubricious silver is to use a silver-containing molecular precursor. These molecules are designed to undergo thermolysis at elevated temperatures, depositing a layer of metallic silver on mechanical surfaces.
In previous work, three generations of silver precursor molecules were evaluated for their performance as extreme temperature additives in motor oil. The Gen-I additive (FIG. 1a) was used to grow low-resistivity metallic silver thin films by aerosol-assisted chemical vapor deposition (AACVD), while the Gen-II additive (FIG. 1b) exhibited promising wear reduction in fully formulated (military grade 15W40) engine oil at ˜200° C. However, it contains phosphorus (P) and sulfur (S) atoms which can poison automotive catalytic converters. The Gen-III additive (FIG. 1c) is a pyrazole-pyridine complex that has the advantage of being P- and S-free, making it benign for use in automotive exhaust systems. However, it requires high wt % loadings (>20%) for effective silver wear and friction reduction, and requires added dimethylsulfoxide (DMSO) for adequate solubility in base oil.
Lower precursor loadings are desirable to reduce the required silver, ensure better solubility, and to accommodate other additives in the additive package. The additive package generally constitutes 10-15% of the entire lubricant formulation. A silver additive that can achieve equal or superior functionality at lower loadings will be necessary to meet the requirements of modern automotive systems.
The present invention seeks to address the problems identified above.